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Herodotus describes the 'Immortals' as being heavy infantry, led by Hydarnes, it provided the professional corps of the Persian armies and was kept constantly at a strength of exactly 10,000 men. He stated that the unit's name stemmed from the custom that every killed, seriously wounded, or sick member was immediately replaced with a new one, maintaining the corps as a cohesive entity with a constant strength.[2]

This elite corps is only called the 'Immortals' in sources based on Herodotus. There is evidence of the existence of a permanent corps from Persian sources, which provided a backbone for the tribal levies who made up the bulk of the Achaemenid armies. These do not however record the name of "Immortals". It is suggested that Herodotus' informant has confused the word anûšiya- ("companion") with anauša- ("immortal"),[2] but this theory has been criticized by Rudiger Schmidt.[1]

During the final decades of the Achaemenid empire, the role expected of the hazarapatish (commanding officer) of the Immortals was extended to include that of chief minister to the king.[3] The provision of a bodyguard, in direct attendance on the monarch, had already been allocated to a select thousand strong detachment of the corps.[3]

Depiction of the "Susian guards" from the Palace of Darius I in Susa. Their garments match the description of the Immortals by ancient authors.[4]

Median (left) and Persian (right) warriors in their ceremonial dress, carvings at Persepolis. Some scholars speculate that they represent the Immortals.

Xenophon (Cyropaedia 6.4.1; 7.1.2) describes the guard of Cyrus the Great as having bronze breastplates and helmets, while their horses wore bronze chamfrons and poitrels together with shoulder pieces which also protected the rider’s thighs.
Herodotus, instead, describes their armament as follows: wicker shields covered in leather, short spears, quivers, swords or large daggers, slings, bow and arrow. Underneath their robes they wore scale armour coats. The spear counterbalances of the common soldiery were of silver; to differentiate commanding ranks, the officers' spear butt-spikes were golden.[2] The regiment was followed by a caravan of covered carriages, camels, and mules that transported their supplies, along with concubines and attendants to serve them; this supply train carried special food that was reserved only for their consumption.[5][6]

The headdress worn by the Immortals is believed to have been the Persian tiara. Its actual form is uncertain, but some sources describe it as a cloth or felt cap which could be pulled over the face to keep out wind and dust in the arid Persian plains.[2] Surviving Achaemenid coloured glazed bricks and carved reliefs represent the Immortals as wearing elaborate robes, hoop earrings and gold jewelry, though these garments and accessories were most likely worn only for ceremonial occasions.[7]

The title of "Immortals" was first revived under the Sassanid army. The backbone of the army was the Aswaran, and the most famous of the Aswaran units were the Zhayedan (literally "Immortals") and numbered 10,000 men, like the Achaemenid predecessors, with the difference that they were heavy cavalry. Their task was mainly to secure any breakthroughs and to enter battles at crucial stages.

Herodotus' account of two warrior elites - the Spartanhoplites and the Immortals - facing each other in battle has inspired a set of rather colorful depictions of the battle, especially in regard of the Immortals:

In the 1962 film The 300 Spartans the Immortals carry a spear and wicker shields like the actual Immortals. However, they are mostly dressed in black and other dark colors, as opposed to historical depictions.[9]

The History Channel documentary Last Stand of the 300 also features the Immortals as part of the reconstruction of the Thermopylae battle. In this version, the tiara the Immortals habitually wear is depicted here as a full-face black cloth mask transparent enough to see through.[9]